Make vendoring libsamplerate and pybind optional

[uis246]

Currently pybind and libsamplerate are fetched unconditionally in external/CMakeLists.txt. This not only ignores versions of those packages installed by package manager and potentially introduces build-runtime version incompatibility, but also breaks build systems that do not allow network access during build time.

[fakufaku]

I have added some comments on the PR.
I agree it may be nice to support locally installed libsamplerate as an option.
However, the current version compiles and statically links libsamplerate. I don't see how that could interfere with the package managers version...